An inkjet printing device is an example of a fluid ejection device that provides drop-on-demand ejection of fluid droplets. A piezoelectric inkjet printer, for example, uses a fluid ejection assembly (i.e., printhead) with a piezoelectric material actuator or element to force fluid droplets out of a nozzle toward a print medium, such as a sheet of paper, to print an image onto the print medium. More specifically, a piezoelectric material actuator includes a flexible piezoelectric material sheet that deforms in response to an applied electric field, generating pressure pulses inside a fluid-filled chamber to eject fluid droplets. Because piezoelectric actuators use pressure instead of heat (e.g., as in the case of thermal resistor actuators) to eject fluid droplets from inkjet nozzles, piezo-based fluid ejection assemblies can accommodate a wide selection of jetable materials.
However, while heat does not limit the use of jetable materials in piezo-based printheads, temperature sensitivity in such printheads remains an issue. For example, short term changes in temperature can cause changes in both fluid drop weight and velocity. More specifically, as temperature increases during a print job, the capacitance of the piezoelectric element increases. Because the driving voltage is typically fixed, the current increases, which heats the piezo-element and the fluid (ink). The increasing ink temperature reduces viscosity at the same time the increasing capacitance strengthens the piezo-element pumping performance. The two effects result in an open-loop system in which drop weight and drop velocity continue to increase, adversely affecting printer performance.
In addition to short term temperature sensitivity issues, longer term degradation can reduce the capacitance of piezo-elements in piezoelectric printheads, resulting in non-uniform performance of the piezo-elements over time. Such degradation can often be seen in non-uniform print patterns that develop in the printed output of piezoelectric inkjet printers over time.